Direct use of the heat of condensation from thermal power plants is uneconomical, particularly over long distances, because of the low temperature of this so-called waste heat having a temperature of about 35.degree. C. The transfer of heat at low temperature requires relatively high volumetric flow of the working medium. As a rule, however, the actual users of the heat are situated far from the locations of large generating plants.
Through appropriate layout and coordination of the power plant components, the heat can be removed at a temperature level desirable for remote heating. In certain thermal power plants, for example, the steam is condensed after discharge from the final turbine stage at a higher pressure, such that, the complete heat of condensation is available for heat utilization, thus sacrificing a substantial portion of the electric power generation, on the order of 30% to 40% of the power generation of the steam generating power plant. The electric energy in this case is a product of steam generation. The maximum power output is available at the maximum heat production, as well as being available at a low heat load, and a lower heat production corresponds to a lower electric power generation.
In other steam generating thermal power plants, where steam is extracted directly fromn the connecting lines, between the medium pressure and low pressure turbine stages, and fed into a heat exchanger, the steam extraction can be adapted to requirements over a wide range of partial load electric power generation. The steam extraction is limited only by the minimum steam volume required for the cooling of the low pressure stage, in a manner such that appropriate pressure relationships are maintained, as well as maintaining the maximum allowable flow rate of the steam volume extraction, and the required steam conditions at the outlet of the medium pressure turbine.
Practically, however, the steam extraction is limited by the electric power load demand at a given point in time, since, as opposed to the former type of plant, the removable heat available decreases in a steam extraction plant when the electric power generated approaches the nominal rated plant capacity.
A basic difficulty, of the combination of electric power generation and heat production for remote heating purposes in thermal steam generating power plants, is the differing transient response time of the heat requirement, and the removable heat available which response depends upon the electric power being generated and which is subject to substantial variations with load. The availability of electric power required is always given priority.
It has already been suggested for the combined generation of electrical power and heat production to install heat reservoirs between the power plant and the remote heating circuit. For example, the heat reservoirs can be charged during low load hours and, during the period of increased heat demand which, for example, corresponds to the hours of peak electric power generation when no extractable heat is available. The stored heat in the reservoirs can then be discharged to the remote heating circuit.
Pressurized reservoirs, for storage of heating water above 100.degree. C., are very expensive, and in practice, are not feasible in the required size. Unpressurized reservoirs, on the other hand, only permit storage at a temperature below 100.degree. C. However, for the efficient operation of a remote heating circuit, particularly in cases of larger distances between the thermal power plant and the remote heating circuit, which must be interconnected with a long distance heat transfer line, a sufficiently high temperature differential between the flow and return, to and from the remote circuit, is essential.
A further disadvantage is that the long distance heating transfer line to the remote heating circuit must be designed for a maximum heat capacity which, however, is only required by the remote user in periods of relatively short duration. If the long distance heat transfer line is only designed for the constant base load of the remote heating circuit, the peak load heat demand must be satisfied by additional heating plants at the end of the long distance transfer line at the rmote heating circuit.
The object, of the present invention, is to provide a method for supplying a remote heating circuit, through a long distance transfer line from a thermal power plant, by using one or more heat reservoirs located close to the power plant, while further providing the capability for generating large amounts of heat sufficient for satisfying peak power demands during a required time period, without affecting the electric power generation, whereby the long distance transfer line, to the remote heating circuit, can always be operated very efficiently at or near its maximum capacity.